Device for producing blown films

ABSTRACT

A device for producing blown films, including an extrusion device generating a tubular film and a flattening device, associated with the tubular film, having two flattening elements, arranged on diametrically opposite zones of the tubular film and adjustable in relation to each other at an acute angle, for transforming the cross-section of the tubular film, which is first circular, then oval, to a double-layer film web. The device further includes a pair of draw-off rolls, mounted downstream of the flattening device, for drawing off the film web. This invention has guide blades that are interposed between the flattening elements and enclose the edge fold which is formed when the tubular film is flattened.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a device for producing blown films out of a thermoplastic plastic.

2. Discussion of Related Art

There are a wide variety of known devices of the above-mentioned type. The functional principle is based on the fact that the melted mass of thermoplastic plastic manufactured by at least one extruder is extruded from an annular exit opening of a die and blown into a tube, which is then folded by a flattening device, conveyed between draw-off rollers, and then supplied to a winding device.

In the simplest case, the flattening device is of two flattening elements, which are arranged in diametrically opposite regions of the tubular film and are adjustable at an acute angle in relation to each other, similar to an inverted V or an A, and whose distance from each other, when viewed in the exit direction of the tubular film, continuously decreases. The flattening elements have a number of guides such as strips, rollers, or brushes, which act on a circumference of the tubular film so that the tubular film, which is initially cylindrical, is gradually flattened, passing through an oval shape with continuously changing diameters, into the form of a double-layered film that is folded at its longitudinal edges.

If the oval cross-sectional shape of the gradually flattened tubular film, which shape takes form starting from the initially cylindrical tubular film, is viewed in the flattening device, then the tubular film, in the region of the larger diameter of the oval cross section at which the folded longitudinal edges of the tubular film finally form, is largely left alone during the flattening because it is not in contact with guide and is not guided by the flattening elements of the flattening device. This can lead to problems with regard to product stability, freedom from wrinkles, and the like.

U.S. Pat. No. 4,170,624 discloses transforming an initially cylindrical tubular film into a rectangular cross section in a special flattening device and then flattening it into a double layer. The intent of this is to avoid mechanical stress on the tubular film during the flattening and resulting changes in the properties profile of the tubular film.

In addition to the flattening elements, to also provide a lateral guidance device with lateral guide elements with guide means guiding the regions of the tubular film that are left free by the flattening elements.

Because of the straight arrangement of the brushes, rollers, or strips used as the guide for the tubular film, which has up to now been the default arrangement in the prior art, it is not possible to laterally guide the tubular film at a plurality of points in an ascending fashion all of the way or distance to the draw-off rollers. On the contrary, one result of the existing form is that only an insufficient point-by-point guidance is possible, which due to the continuously changing cross section of the film web and the continuously decreasing available space between the flattening elements, must end or terminate a certain distance from the draw-off rollers. The forming edge fold of the tubular film thus remains unguided. Particularly in reversing movements of the flattening device and/or a withdrawal device for the tubular film, which is situated after the draw-off rollers, the film starts to form slight wrinkles that run all the way through the completed roll and reduce the quality. This situation is clearly in need of improvement.

SUMMARY OF THE INVENTION

In order to solve this problem, this invention proposes situating guide blades on both sides of the tubular film, which extend between the flattening elements and preferably to the draw-off rollers, enclose the respective edge folds of the tubular film that form during the flattening of the tubular film, and guide them in a definite fashion into a nip between the draw-off rollers.

Correspondingly, the guide blades have a three-dimensional embodiment adapted to a contour of the tubular film as it undergoes the flattening process and allow the double-layered film web to be fed between the draw-off rollers in a smooth manner.

The guide blades can be produced from suitable plastics or also aluminum, for example, if they come into contact with the tubular film or the guide blades can also be equipped with a porous, preferably microporous, surface oriented toward the tubular film to permit a supplied current of air to emerge and produce an air cushion for a contact-free guidance of the tubular film.

In the context of the present invention a “microporous surface” is understood to be one with an average pore size of 5 to 100 μm.

A distance of the guide blades according to this invention from the subsequent draw-off rollers should be selected to be small and is preferably adjustable, thus preferably permitting a definite guidance of the edge fold of the tubular film all the way into the roller nip of the draw-off rollers. If possible, the edge fold is guided until close to or immediately before or prior to the draw-off rollers, such as the smallest possible distance from the squeezing rollers.

Preferably, the guide blades according to this invention, viewed in the exit direction of the tubular film, form the upper extremity of lateral guide elements of a lateral guide device, which are situated or positioned between the flattening elements of the flattening device and guide the circumference regions of the tubular film that are not currently being acted on by the flattening elements, such as the gradually forming edge fold and the adjacent regions of the tubular film.

According to one embodiment of this invention, the guide blades are secured to the lateral guide elements in an adjustable fashion. Both the distance relative to the lateral guide element and thus to the subsequent draw-off rollers and the angular orientation in relation to the lateral guide element can be adjusted and in addition, the horizontal position can be adapted to different diameters of the tubular film.

In this case, the lateral guide elements can be of a support structure with adjusting means and support arms fastened to the support structure, which support the guide means, fastened to holding forks for example, for the tubular film.

The guides themselves can be rollers or drums of a suitable plastic such as CFK, silicone, or also aluminum; it is also possible to provide roller brushes or to provide plates with a microporous surface from which a current of air emerges in order to produce an air cushion between the plate and the tubular film, to ensure a contact-free guidance. In the case of rotating rollers or drums, it is preferable to ensure a particularly smooth running.

The support structure is preferably of a plurality of support legs, which are connected to one another in articulating fashion and can be individually adjusted by suitable adjusting drive units in accordance with the desired path of the tubular film as it passes through the flattening device. In this case, one or more support arms for holding guides can be fastened to each support leg. At least one support arm is provided for each support leg.

Other adjusting possibilities can be provided by securing the guide to the support arms, in a way that allows them to pivot around a pivot axis extending in the exit direction of the tubular film.

The adjustment of the support legs, which are connected to one another in articulating fashion and extend from one another in segmented fashion, can be embodied in various ways, not only with respect to the angle enclosed between adjacent support legs, but also with respect to the horizontal distance from the respective opposing lateral guide element in order to adapt to the diameter of the tubular film. According to another embodiment of this invention, it is also possible for the individual support legs to be longitudinally adjustable, such as by a suitable telescoping mechanism, which offers additional degrees of freedom for adapting to the desired contour of the tubular film to be flattened.

By embodying the lateral guide element proposed according to this invention with a large number of support legs and a guide blade at the upper end of the latter, it is possible to emulate arc-shaped desired contours of the tubular film with a high degree of approximation, particularly if the support arms fastened to the individual support legs and the holding forks, which are for the guide and are fastened to these arms, are adjustable, for example vertically adjustable, laterally adjustable, and angularly adjustable.

The guides are preferably situated in the vicinity of the lateral guide elements provided according to this invention so that they support the tubular film over the largest possible regions of its circumference and, together with the flattening elements, support the tubular film in a definite, three-dimensional fashion.

The adjustment of the individual support legs can, for example, be executed with a high degree of positioning precision by spindle drive units or other suitable drive units. It is also preferable for each support leg to have a separate adjusting drive unit. The adjusting drive units can also be centrally connected to one another.

BRIEF DESCRIPTION OF THE DRAWINGS

Other embodiments and details of this invention are explained in view of an exemplary embodiment shown in the drawings, wherein:

FIG. 1 is a schematic side view of a device according to one embodiment of this invention;

FIG. 2 shows a cross-sectional shape of a tubular film that changes as the tubular film is flattened; and

FIG. 3 is a view of a part of a lateral guide element embodied according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic view of a tubular film 1, which is manufactured in a known manner, guided vertically upward in an exit direction A out of an extrusion device, gradually folded into a flat tubular film by flattening elements, not shown in detail, of a flattening device, and pulled through a nip between parallel-mounted draw-off rollers 5. The drawing also shows a lateral guide element 2, which is part of the flattening device and is also provided on the opposite side, symmetrical to the central longitudinal axis of the tubular film 1.

Through the action of the flattening elements, which according to the depiction in FIG. 2, engage a region labeled with the reference numeral 300 of the initially cylindrical tubular film 1, such as with a circular cross section Q1, the tubular film 1 is gradually transformed, passing through oval cross sections that are labeled Q2 and Q3, by way of example, into a flattened double-layered film web with a cross section Q4.

During this flattening process from the cross section Q1 to the cross section Q4, the lateral guide elements 2 guide the tubular film 1 continuously in the region 400 of the larger diameter, such as between the flattening elements labeled with the reference numeral 300.

Each lateral guide element 2 comprises, according to FIG. 1, a support structure with three support legs labeled with the reference numerals 20, 21, 22, which are connected to one another, such as in a chain, by articulating joints 23, 24.

Each of the support legs 20, 21, 22 supports support arms 25 to which are secured guides or guide means 27 for the tubular film 1, which are oriented toward the tubular film 1, are embodied in the form of CFK rollers, for example, and contact the surface of the tubular film 1, guiding it precisely in the desired contour.

As shown in FIG. 3, the free end of each support arm 25 can have pairs of guide means 27 that are supported, if necessary, in an articulating fashion by a pivot axis S extending in the entry direction of the tubular film 1 in order to adapt optimally to the circumference of the tubular film 1.

In lieu of guide means in the form of rollers, the upper support segment 20, viewed in the exit direction A, has a guide blade 27 a that encloses the outside of the edge fold 10, which forms during the flattening of the tubular film 1, and ensures a precise guidance of it all the way to the draw-off rollers 5. The guide blade 27 a is adjustable with regard to a distance from the draw-off rollers 5 and an angular orientation in relation to the support leg 20 and guides the edge fold 10 of the tubular film precisely into the nip between the draw-off rollers 5, which are positioned or situated congruently one behind the other in the drawing.

Because of the articulating connection between the individual support legs 20, 21, 22, it is possible to adjust them relative to one another with respect to the enclosed angle in order to adapt to the desired contour of the tubular film. For this purpose, independent drive units such as spindle drive units 28 are provided, which in the exemplary embodiment shown, engage the support leg 20 and the support leg 22 while the support leg 21 positioned between them has a cantilever 29 that is engaged by the drive unit 28. The drawing also shows an additional engaging arm of a drive unit 28 a, which directly engages the cantilever 29. With this configuration and through actuation of the individual drive units 28, 28 a, it is possible within broad boundaries to adjust the angular orientation of the succeeding support arms and thus of the guide means for the tubular film 1 that are fastened to them and in particular, it is possible to optimally reproduce the arc-shaped edge curvatures that form on the tubular film 1 to be flattened and thus to achieve a significantly more precise guidance that results in a significantly improved quality of the flattened tubular film 1.

Alternatively, it is also possible to provide a separate drive unit 28 for each support leg 20, 21, 22, without connecting these drive units to one another.

The support legs 20, 21, 22 can be adjusted independently of each other in the direction of the diameter D of the tubular film and can also each telescope in the arrow direction T, for example are embodied as longitudinally adjustable.

In lieu of the three support legs 20, 21, 22 shown here, it is also possible to provide only two such support legs or more than three support legs and to provide them with corresponding drive units. In another embodiment, each support leg has at least one support arm 25 for a guide means. 

1. A device for producing blown films, comprising an extrusion device that produces a tubular film (1) and having a flattening device associated with the tubular film (1) that has two flattening elements situated in diametrically opposite regions of the tubular film and that are adjustable at an acute angle in relation to each other so that an initially circular cross section of the tubular film is transformed, passing through oval cross sections, into a double-layered film web, and having a pair of draw-off rollers situated after the flattening device for drawing off the tubular film, between the flattening elements guide blades (27 a) enclose respective edge folds of the tubular film (1) which form during flattening of the tubular film (1), and guide the edge folds into the nip between the draw-off rollers.
 2. The device as recited in claim 1, wherein the guide blades (27 a) are positionable against the tubular film (1) or, the guide blades (27 a) can produce an air cushion for a contact-free guidance of the tubular film (1).
 3. The device as recited in claim 2, wherein lateral guide elements (2) for regions of the tubular film left free by the flattening elements are situated between the flattening elements and the guide blades (27 a) are situated at an upper end of the lateral guide elements (2), viewed in an exit direction (A) of the tubular film (1).
 4. The device as recited in claim 3, wherein the guide blades (27 a) are adjustably secured to the lateral guide elements (2).
 5. The device as recited in claim 4, wherein the guide blades extend until close to the draw-off rollers.
 6. The device as recited in claim 1, wherein lateral guide elements (2) for regions of the tubular film left free by the flattening elements are situated between the flattening elements and the guide blades (27 a) are situated at an upper end of the lateral guide elements (2), viewed in an exit direction (A) of the tubular film (1).
 7. The device as recited in claim 6, wherein the guide blades (27 a) are adjustably secured to the lateral guide elements (2).
 8. The device as recited in claim 1, wherein the guide blades extend until close to the draw-off rollers. 